1. Field of the Invention
The present invention relates to a video evaluation apparatus, spatio-temporal frequency analyzing apparatus, video evaluation method, spatio-temporal frequency analyzing method, video evaluation program, and spatio-temporal frequency analyzing program for evaluating a video.
2. Related Background of the Invention
A “video” is composed of a sequence of “frames” of single still images. A target of video processing including acquisition, accumulation, transmission, display, coding, decoding, and so on of a video is generally a video in which spatial resolutions of individual frames (hereinafter referred to simply as resolutions) are fixed or gradually varied. The human eyes are able to clearly discriminate the differences of the resolutions of images. Accordingly, the differences of resolutions are often connected directly with evaluation on qualities of images, and images of high resolutions are preferred in general.
The document below describes occurrence of the following visual illusion: even in the case where individual frames are images of a low resolution, when they are displayed as a sequential video of the frames, the video looks to a person viewing it as if it were a clear video with an enhanced resolution of images, and is perceived as smoother motion. This visual illusion is called motion sharpening.
Takeuchi T, Karen K. De Valois, “Sharpening image motion based on spatio-temporal characteristics of human vision,” Human Vision and Electronic Imaging X, Mar. 18, 2005; Volume 5666, No. 5666, p 83-94; ISBN/ISSN: 0-8194-5639-X
The above document describes that the motion sharpening can arise from the magnitude of motion of a subject in images and from high-frequency components of spatial frequencies. It is also described as to the motion sharpening that when frames of a high resolution are interposed between images composed of low-resolution frames and when they are viewed as a motion picture, the resolution of the low-resolution frames is perceived as being higher than the original resolution. Particularly, where 50% of frames constituting a video are high-resolution frames, the resolution of the video seems the same as that of a motion picture consisting of all high-resolution frames, i.e., the video appears such that the perceived resolution is constantly high.
On the other hand, it is known as to the human visual features that the sensitivity of eyes has characteristics for spatio-temporal frequencies of the video (hereinafter referred to as spatio-temporal frequency characteristics). Namely, there exists a property that the human eyes are more sensitive to specific spatio-temporal frequencies contained in images. It is also known that the sensitivity of eyes as a human visual feature becomes lower with increase in spatio-temporal frequencies of a video viewed. It is thus considered that even with change in spatio-temporal frequencies of the video, the lowered sensitivity of eyes can lead to failure in perceiving the difference.